I believe I've found the way to find a blank cell, now got to put it in a loop to run through rows but for anyone whose interested. the code is:
int i = 0;
while(((excel.Range)xlWorkSheet.Cells[i,"Y"].Value2 != null)
{
i++;
}
xlWorkSHeet.get_Range("Y"+i,misValue).Value = landfill.Value;
May help someone else out :)